Silicate binders for zinc-rich paints

ABSTRACT

New compositions of matter are made available which contain silicate esters and an alkaline material, the mixtures being kept anhydrous until their use. The compositions are zinc-rich paints, a silicate ester and an alkaline material mixture being mixed with zinc dust. Upon use, the blend is spread out as a coating. Upon exposure to the moisture in air, drying occurs rapidly with the formation of very hard finishes that greatly resist cutting by sharp edges.

United States Patent Oken [ 51 May 2, 1972 [54] SILICATE BINDERS FORZINC-RICH PAINTS [72] Inventor: Aaron Oken, RD. 02, Sills Mills Road,

Kennett Square, Pa. 19348 [22] Filed: Apr. 14, 1970 211 Appl. No.:28,551

Related US. Application Data [63] Continuation-impart of Ser. No.736,948, June 14,

1968, abandoned.

[52] U.S.Cl ..l06/1, 106/14, 106/287 SE, ll7/l3l, ll7/l35.l, ll7/l60R [51] Int. Cl. ..C09d 5/10 [58] Field ofSearch ..l06/l, l4, 38.3, 38.35,287 B, 106/84 M; 117/1351, 131, 160

[56] References Cited UNITED STATES PATENTS 3,202,517 8/1965 Jarboe eta]. l06/l4 3,232,771 2/1966 Pearce l 06/ 38.35

FOREIGN PATENTS OR APPLICATIONS 163,467 6/1955 Australia 106/287 612,6221 H1948 Great Britain 106/287 647,537 12/1950 Great Britain 106/287Primary Examiner-Lorenzo B. Hayes Attorney-C. Walter Mortenson [5 7]ABSTRACT 7 Claims, No Drawings SlLlCATE BINDERS FOR ZINC-RICH PAINTSThis is a continuation-in-part of my application Ser. No.

7 736,948, filed on June 14, 1968, now abandoned.

This invention deals-with an improved silicate binder for particulatematter which is especially useful in the preparation of zinc-richpaints. lt also deals with a storage stable zinc-rich paint comprising amixture of a catalyst, zinc, and ethyl silicate, which mixture is highlyreactive to atmospheric moisture and cures rapidly when exposed to theair. More particularly, this invention deals with the provision ofhermetically sealed zinc-rich paints.

It is well known that ethyl silicate will react with water and hydrolyzeto silica, as follows:

and it has been proposed that the silicate material be used either byitself or with added pigments and fillers as a paint. However, in theabsence of acid or alkaline catalysts, water and ethyl silicate reactvery slowly, and by itself ethyl silicate makes an impractical paint. lnthe past, this slow conversion to silica has been accelerated by firstpartially hydrolyzing the ethyl silicate with a limited amount of.water. When this hydrolysis is carried out with an acid catalyst,usually hydrochloric acid, a fairly stable solution of partiallyhydrolyzed ethyl silicate can be obtained. When this partiallyhydrolyzed ethyl silicate containing the acid catalyst is spread out ina thin film the absorption of relatively small amounts of atmosphericmoisture completes the hydrolysis of the ethyl silicate and this thenfully hydrolyzed ethyl silicate (or silicic acid) condenses to silica ina relatively short time.

However, there is a practical limit as to how much this conversion canbe accelerated by prehydrolyzing the ethyl silicate; the more completelythe ethyl silicate is prehydrolyzed the more rapid is the subsequentconversion to silica on exposure to the air but the more unstable is theprehydrolyzed ethyl silicate to storage since it tends to convertspontaneously to silica on standing and the shelf life is shortened.

In practice it has been found that this partially hydrolyzed ethylsilicate can be used to make very good zinc-rich paints for protectingsteel from corrosion. When this partially hydrolyzed ethyl silicate ismixed with zincdust and applied to a steel surface, the final coating, amixture of zinc, silica and their various reaction products such as zincsilicate, is a hard, stone-like coating with excellent anti-corrosionproperties.

Unfortunately, the hardening of these zinc-rich paints requires optimumconditions of temperature and humidity which are seldom encountered inactual field conditions. in practical use, the setting up of the paintis often slow and erratic and the properties of the final coating areuncontrollably variable.

Moreover, all presently known zinc-rich paints based on partiallyhydrolyzed ethyl silicate are two component systems: that is, they arepackaged with the zinc dust and the silicate solution in separatecontainers that must be mixed just prior to use since, under theinfluence of the acid catalyst, the partially hydrolyzed ethyl silicatewill react with the zinc and the paint will rapidly set up in thecontainer to a solid mass.

Thus, an objective of this invention is the provision of compositions ofmatter or articles of manufacture which overcome the aboveproblems. Afurther aim is the providing of stable compositions which have longshelf lives. A still further goal is the provision of a silicate mixturewhich can be stored in a container in an unhydrolyzed state for longtime periods but which, upon opening the container, can be used as acoating material to produce hard, protective coatings. These and otherpurposes of this invention will appear hereinafter.

It is well known that ethyl silicate reacts extremely rapidly withalkaline solutions and is converted directly to silica. l have found,however, that an anhydrous solution of ethyl silicate and an alkalinematerial is stable and shows an indefinite shelf life provided thatmoisture is excluded from the mixture.

When a thin film of this anhydrous mixture is exposed to atmosphericmoisture, the liquid film can be converted to silica in as short a timeas 10 minutes. In comparison, an uncatalyzed film of ethyl silicate willremain liquid for several weeks and films of acid catalyzed, partiallyhydrolyzed ethyl silicate require several hours to solidify.lmportantly, l have also found that zinc dust may be added to theanhydrous silicate mixture and that the anhydrous composite when sealed,preferably hermetically, is stable over long periods of time, beingusable as a coating material when desired.

The most effective alkaline catalysts for use in this invention arethose which can be brought into solution with ethyl silicate in ananhydrous solvent. For example, anhydrous solutions of sodium hydroxideor, still better, sodium methoxide in methanol are excellent catalysts.Magnesium ethoxide on the other handv is not completely soluble in analcoholic ethyl silicate mixture and is less effective as a catalyst.Finally, particulate alkaline materials such as trisodium phosphatesuspended in ethyl silicate are only slightly effective catalysts: suchmixtures require several days to harden. It is preferred that thecatalyst be present in a dissolved state.

it is an important feature of this invention that the ethyl silicatesolution with the alkaline catalyst be anhydrous. In this way, a stablemixture is obtained which shows an indefinite shelf life. It is onlywhen this mixture is exposed to moisture that the hardening reactionbegins.

This anhydrous mixture has another important practical advantage.Although it is well known that zinc is rapidly attacked by alkalinesolutions, under the anhydrous conditions of this invention zinc andalkali are unreactive and the paint can be packaged as a completelymixed one-container system with an indefinite shelf life. I havefoundthat, properly formulated, such mixtures of ethyl silicate, zincand an alkaline catalyst will dry rapidly under any conditions oftemperature and humidity that are likely to be encountered in actualuse. Even at below freezing temperature and very low humidity such filmswill be touch dry in 1 hour and have the unusually high scratch and marresistance of zinc-rich silicate coatings in 2 to 3 hours. Under typicalsummer atmospheric conditions the coating will be touch dry in 15minutes and essentially completely hard in 1 hour. These coatingsprotect steel against fresh and salt water corrosion at least as well asthe more conventional types of zinc-rich paints.

Although all the foregoing has mentioned ethyl silicate as the basismaterial, this invention is not limited to simply" tetraethylorthosilicate. The same results are obtained with partially polymerizedforms of ethyl silicate. These materials are known in commerce ascondensed ethyl silicates. In fact, the preferred form of ethyl silicatefor this invention is a polymerized product which contains 40 percent ofsilica and is known technically as ethyl silicate 40. Furthermore, it isto be understood that the products of this invention are not limited tothe simple solutions so far described. In addition to the ethylsilicate, zinc and alkaline catalyst one can add a variety of materialsto achieve certain desired purposes. For example, one may add suspendingagents such as bentonite clays to help disperse the zinc dust, and onemay add various inert pigments to color the final coating. Furthermore,these anhydrous, alkaline solutions of ethyl silicate are not limited touse only with zinc. One can obtain excellent completely inorganiccoatings by adding titanium dioxide or other alkali stable pigments tothe solution.

The following examples illustrate the invention but are not Iirnitative:

EXAMPLE I Zinc dust (600 g.) was added under anhydrous conditions, as,for example, by operating under dry nitrogen, to a mixture of 124 g. ofethyl silicate 40, 40 g. of a 25 percent solution of sodium methoxide inmethanol and 25 cc. of Cellosolve in a high shear stirrer. Within 5minutes a smooth paint was formed which was stored in a tightlystoppered jar. When this freshly prepared paint was coated onto asandblasted steel panel and exposed to the atmosphere at F. and 82percent relative humidity, the coating was dry to the touch in 1 hour.After 24 hours the coating resisted scratching with a sharp EXAMPLE IlZinc dust (600 g.) was added under anhydrous-conditions as above to amixture of 124 g. of ethyl silicate "40, 20 g. ofa

25 percent solution of sodium methoxide in methanol, 20 cc. of anhydrousmethanol and 25 cc. of Cellosolve in a high shear stirrer and blended toa smooth paint. When the freshly prepared paint was exposed to theatmosphere at 85 F. and 82 percent relative humidity, the coating wasdry to the touch in l and 15 hours and seemed completely hard after 24hours.

Again, after storage for several months, similar coatings are obtainedfrom the stored mixture.

EXAMPLE Ill EXAMPLE IV Zinc dust (600 g.) was added under anhydrousconditionsto a mixture of 124 g. of ethyl silicate 40 and 100 g. of a 10percent solution of potassium hydroxide in anhydrous ethyl alcohol in ahigh shear stirrer. Within 5 minutes a smooth paint was formed which wasstored in a tightly stoppered jar. When this paint was coated onto metaland exposed to the atmosphere, the coating was dry to the touch in l anda hours and after 24 hours was completely hard.

EXAMPLE V Sodium hydroxide (l g.) was ball milled with 50 g. of toluenefor 8 hours to break down the sodium hydroxide into a fine powdersuspended in the toluene. This mixture of sodium hydroxide and toluenewas added to 600 g. of zinc dust and 124 g. of ethyl silicate 40 in ahigh shear stirrer and milled to give a smooth paint. When this freshlyprepared paint was coated onto metal panels and exposed to theatmosphere, the film became dry to the touch in 4 hours and after 48hours was completely hard. After 4 months of storage, this compositionwas again coated onto metal panels and exposed to the atmosphere. After4 hours the coating was dry to the touch and was hard after 48 hours.

The critical proportions of ethyl silicate and alkali which will giveuseful products fall in the range of 1 percent of inorganic alkali byweight based on the weight of the silicate as the lower limit to 15percent as the upper limit. The preferred range is within the ratio -10percent of alkali, and the preferred silicate is ethyl silicate. Theratio of zinc to total solids (zinc and silica) can very widelydepending on the anticipated use of the paint. For most applications thezinc content will be from 60-95 percent, with the preferred range from80-92 percent, of the total solids.

The silicate is normally tetraethoxy silane (ethyl silicate) obtainablecommercially generally as a mixture which includes ethyl polysilicateswhich occur from partial hydrolysis and condensation reactions. Whileethyl silicate is preferred, other silicyl esters may be used includingother lower alkoxy silanes, aryloxy silanes and aralkoxy silanes suchsilanes containing methyl propyl, butyl, ethyl-hexyl, phenyl, benzylgroups, orthe like, as desired. Commercially, ethyl silicate is the onlyimportant one, and it is obtainable as tetraethyl orthosilicate,condensed ethyl silicate and'ethylsilicate 40,

the latter, because of its high silica content, being preferred. Thesilica content usually is in the 40-45 percent range and the industryswide use of zinc silicates is generally based on ethyl silicate having40-45 percent silica content.

The catalysts used in this invention include sodium methoxide, sodiumethoxide, potassium methoxide, potassium ethoxide, sodium hydroxide andpotassium hydroxide, lithium hydroxide and lithium methoxide, lithiumethoxide, lithium isopropoxide, lithium butoxide, as well as the similaralkoxides -of sodium-and potassium. In practice, sodium methoxide ispreferred for its low cost and availability. Solution of the catalystsin the silicate is effected by using a solvent in which the catalyst andthe silicate are soluble. Such solvents are the lower alcohols such as"methanol, ethanol, propanols and Cellosolve. While other solvents can beused, these are cheap and effective materials and are convenientlyplaced in the an-v hydrous form needed for the purposes of thisinvention. While ethyl alcohol is produced as a product during thesetting reactions, it is to be appreciated that the intimate contact ofthe silicate and the catalyst is needed to effect that production and,therefore, solution is effected before the curing is to begin. Thus,solutions of the alkaline catalysts varying in solids content from about1 percent to about 30 percent are used, with a 25 percent concentrationusually being used. Suspension of alkaline catalyst can be effected in awide variety of solvents such as benzene, toluene, the alcohols namedabove and the like.

Anhydrous conditions are obtained and maintained in any of theconventional ways. Starting materials, such as methanol and thesilicate, are readily placed in anhydrous condition as, for example, bydrying them in conventional, commercial dryers to mention only onemethod. The mixing steps are generally done under a blanket of the inertgas. While a number of such gases can be used, dry nitrogen isordinarily used because of its ready availability and low cost. While itis possible to add to the compositions of this invention materials whichwill react with or otherwise destroy or remove water, should water vaporget into the compositions of this invention prior to their use,generally such preventive steps are not needed. One reason for this isthat the compositions of this invention are usually packaged withresidual nitrogen gas still within the mass, so that the packagedproduct isactually under a slight pressure. Further, the packaging artis quite highly developed so that precision and very tight seals arevery readily obtained with the result that the probabilityvof leakageand exposure to humid air prior to use is very slight. Thus, themixtures of this invention usually can be poured into a container andsimply sealed, since they usually contain a solvent the evaporation ofwhich just prior to sealing leads to the formation of a hermetical sealsince air is driven out of the container by the evaporation.

As already mentioned, these alkali catalyzed mixtures of silicate estersand zincdust are valuable not only for the fact that the anhydrousmixtures are storage stable but also for the fact that they react veryrapidly with moisture. For example, under typical summer atmosphericconditions, the coatings of the composition of this invention will dryto the touch in l 5 minutes and be completely hard in 2-3 hours whileacid catalyzed mixtures of partially hydrolyzed ethyl silicate willrequire several hours before they are sensibly hard, and uncatalyzedethyl silicate by itself will remain fluid for several weeks.

While this invention has been described with particular reference tozinc silicates, it is to be understood that the principles of thisinvention apply also to zinc-lead silicates, zinc phosphate silicatesand zinc titanate silicates and to silicates that contain inertfinelydivided materials such as pulverized polymers, pulverized andother'forms of clays, pigments and the like, as, for example. ironoxide, chromium oxide, carbon black, titanium oxide, ocher and rawsienna, and silica may be added as suspending agents and thickeners.

The products of this invention may be used not only in creatingprotective coatings for all sorts of surfaces, metal and non-metal, butin bonding and in impregnating procedures.

It is well known that if a mixture of zinc dust, ethyl silicate orpartially hydrolyzed ethyl silicate, and an acid catalyst is applied asa coating to metal surfaces and exposed to atmospheric moisture, waterabsorbed from the air will convert the ethyl silicate into a silicabinder for the zinc dust and form a hard, impervious, corrosionresistant coating on the metal. By itself, however, water and ethylsilicate react too slowly to give a useful coating and it is thefunction of the acid catalyst, usually hydrochloric acid, to acceleratethis reaction to provide a practical composition. However, this acidcatalyzed mixture is not storage stable since various reactions can takeplace between the components. For example, the acid reacts with the zincdust to produce hydrogen gas and zinc ions. The zinc ions and thepartially hydrolyzed ethyl silicate in turn react to form zinc silicate.As these reactions proceed, the mixture sets to a solid mass in the can.For this reason, all conventional zinc-rich paints are sold as twocomponent mixtures consisting of a container of zinc dust and a separatecontainer of ethyl silicate and hydrochloric acid. These components mustbe mixed just before application since the pot life of the completemixture is measured in hours. Moreover, even the acid catalyzed reactionof ethyl silicate and water is rather slow and, as a result, the rate atwhich the coating hardens is dependent upon the temperature and humidityof the environment. This variability in the drying time of conventional,acid catalyzed coatings produces coatings which show widely variableproperties of corrosion resistance.

As shown above, use of certain inorganic alkaline catalysts allows oneto prepare a single container mixture of zinc and ethyl silicate whichis not only highly reactive to atmospheric moisture but, even moresurprising, is indefinitely stable in the sealed can. Thesecompositions, which will harden in minutes when exposed to atmosphericmoisture, remain unchanged for at least 1 year when stored in ananhydrous condition.

It is well known that mixtures of amines and ethyl silicate will givestable mixtures which harden rapidly when exposed to moisture. Inaddition, it is known that various inert materials such as sand and claycan be added to this mixture of ethyl silicate and amines with no illeffects upon the storage stability of the anhydrous systems. However, Ihave found that mixtures of amines, ethyl silicate and zinc dust do notgive stable mixtures. Such a mixture on standing at room temperaturestarts to develop a yellow color after a few weeks time. After 4 to 6weeks, the mixture is dark brown and is no longer reactive to moisture.For this reason, the use of amines is to be avoided in the preparationof shelf stable, single container zinc-rich paints.

Quite surprisingly, I have found that zinc dust, ethyl silicate and verystrong inorganic bases such as sodium hydroxide or sodium methoxide areshelf stable for at least 18 months and after this time are still veryreactive to moisture and form coatings which harden rapidly on exposureto the air.

The most effective catalysts for this purpose are solutions of alkalimetal alkoxides in alcohols. These may be prepared by adding sodium tovarious alcohols such as methanol or ethanol, for example, or bydissolving pre-forrned sodium methoxide in alcohols, or by dissolvingsodium hydroxide in alcohols. Finely divided sodium hydroxide is also aneffective catalyst although its effectiveness may be due at least inpart to the fact that the hydrolysis of the ethyl silicate furnishesethyl alcohol which can react with the hydroxide to form sodium ethoxidein situ.

While this invention has been described with particular reference tozinc and silica, it is understood that the principles of this inventionencompass a broad range of materials that can be used as inert orreactive fillers in the silicate binder. For, example, this inventioncan also be applied to mixtures of zinc and lead with silicate, or withthe metal wholly or partially in the form of oxides such as zinc andlead oxide, or as phosphates, silicates, titanates, etc. In addition, itis understood that the principles of this invention are not limited tothe application of coating to metals; with appropriate fillers andpigments, storage stable but'highly reactive compositions can beprepared which show excellent adhesion to ceramics, cement, stone andglass, and such compositions are to be included in the scope of thisinvention.-

While the invention has been disclosed herein in connection with certainembodiments and certain structural and procedural details, it is clearthat changes, modifications or equivalents can be used by those skilledin the art: accordingly, such changes within the principles of theinvention are intended to be included within the scope of the claimsbelow.

Iclaim:

1. In a zinc-rich paint comprising zinc dust and an anhydrous vehiclecomprising an alkyl polysilicate, the improvement comprising an alkalimetal alkoxide in said paint in an amount sufficient to catalyze thehardening of said paint.

2. The paint in accordance with claim 1 in which said vehicle alsocontains an alcohol.

3. The paint in accordance with claim 2 in which said alcohol isethanol.

4. The paint in accordance with claim 1 in which said alkoxide is sodiumethoxide.

5. The paint in accordance with claim 1 in which said amount of saidalkoxide is about 1 percent to about 15 percent based on the weight ofsaid silicate.

6. The paint in accordance with claim I in which said zinc dust ispresent in amounts of about 60 percent to about percent based on thetotal solids in said coating.

7. The paint in accordance with claim 1 in which said alkali metal isselected from the group consisting of sodium, potassium and lithium.

2. The paint in accordance with claim 1 in which said vehicle alsocontains an alcohol.
 3. The paint in accordance with claim 2 in whichsaid alcohol is ethanol.
 4. The paint in accordance with claIm 1 inwhich said alkoxide is sodium ethoxide.
 5. The paint in accordance withclaim 1 in which said amount of said alkoxide is about 1 percent toabout 15 percent based on the weight of said silicate.
 6. The paint inaccordance with claim 1 in which said zinc dust is present in amounts ofabout 60 percent to about 95 percent based on the total solids in saidcoating.
 7. The paint in accordance with claim 1 in which said alkalimetal is selected from the group consisting of sodium, potassium andlithium.